FIG. 1 illustrates an equivalent circuit of a typical electromagnetic induction-type wireless power transfer system using respective resonant coils of a transmitter and a receiver. When the self-inductance, resistance, and capacitance for resonance of a transmitting coil (Tx coil) are L1, R1, and C1, respectively, and the self-inductance, resistance, and capacitance for resonance of a receiving coil (Rx coil) are L2, R2, and C2, respectively, the power of the Tx coil that receives an Alternating Current (AC) source Vs may be transferred to a load (impedance ZL) connected to the Rx coil depending on magnetic coupling based on mutual inductance M12. The load may include a rectifying circuit, DC-DC converter, a controlling circuit, a battery charger, etc. A power transmission unit including the Tx coil is provided in a transmitter for power transmission, and a power reception unit including the Rx coil is provided in various types of electronic devices that consume power, such as a smart phone and an iPad. An electronic device may be located close to the power transmission unit and allow power to be supplied to the load of the electronic device in a wireless manner through the Rx coil of the electronic device.
However, in the wireless power transfer system, the intensity of magnetic coupling between the above-described Tx and Rx coils varies with the structures, geometrical arrangement, and positions of the Tx and Rx coils and a distance between the Tx and Rx coils. When the intensity of magnetic coupling between the transmitting and receiving coils varies depending on various environmental changes, the optimal power transfer condition of a wireless power transfer system changes. Thus, complexity is required in such a way that an additional impedance matching circuit is provided in the transmitter or receiver to satisfy the condition of the maximum power transfer, or a current voltage sensing circuit or the like is provided so as to control the optimal power transfer condition. In particular, when a plate-type transmitter is used, optimal impedance matching between the transmitter and the receiver must be realized depending on the position of the receiver placed on a plate, and thus a problem arises in that it is difficult to support wireless power transmission at a free position (free positioning) between the transmitter and the receiver. Further, when mutual inductance between the transmitting and receiving coils differs for each position, or when multiple devices are located at different positions to receive power, a problem arises in that it is difficult for the transmitter to perform impedance matching to respective devices for different impedances, thus making it impossible to simultaneously support power transmission to multiple devices.
In relation to conventional wireless power transmission technology for maximum power transfer between a transmitter and a receiver, various documents are published, and the following four documents among the documents are introduced below.    (1) A. Kurs, A. Karalis, R. Moffatt, J. D. Joannopoulos, P. Fisher, and M. Soljacic, “Wireless power transfer via strongly coupled magnetic resonances”, Science, vol. 317, pp. 83-86, July 2007.    (2) J. Park, Y. Tak, Y. Kim, Y. Kim, S. Nam, “Investigation of adaptive matching methods for near-field wireless power transfer”, IEEE Transactions on Antennas and Propagation, vol. 59, pp. 1769-1773, May 2011.    (3) W. S. Lee, H. L. Lee, K. S. Oh, and J. W. Yu, “Uniform magnetic field distribution of a spatially structured resonant coil for wireless power transfer”, Applied physics Letters 100, 2012.    (4) W. S. Lee, W. I. Son, K. S. Oh, and J. W. Yu, “Contactless energy transfer systems using antiparallel resonant loops”, IEEE transactions on industrial electronics, Vol. 60, No. 1, January 2013.
In the above document (1), a maximum power transfer condition that varies with a change in distance has been satisfied by additionally using transmission/reception coupling coils in addition to transmission/reception resonant coils. However, this method is problematic in that it is difficult to apply such a method to a limited space when the limited space is used because the added transmitting/receiving coupling coils must be physically moved.
The above document (2) discloses a configuration that uses the splitting of transmission/reception penetration characteristics appearing while coupling changes according to the distance, as a method of tracking an optimal frequency so as to transfer the maximum power according to the distance between the transmitting/receiving resonant coils. However, there is a problem in that, when the frequency for near-field wireless power transfer is fixed, it is difficult to use such a frequency tracking method.
The above document (3) proposes a structure of having a uniform magnetic field distribution at a predetermined height of a rectangular coil by bending the rectangular coil, but it is disadvantageous in that the shape of the coil must be mechanically deformed. Also, there is a disadvantage in that, unless the receiver has a uniform magnetic field, uniform mutual inductance or a uniform figure of merit cannot be obtained.
The above document (4) discloses a method for maintaining mutual inductance that varies according to the distance between transmitting and receiving coils by utilizing two series-connected loop coils through which currents flow in opposite directions. This may be utilized only when the central axes of transmitting/receiving coils are aligned with each other, but it is difficult to freely position a receiver on a plate-shaped transmitter, and it is also difficult to charge multiple receivers placed on the transmitter in a wireless manner.